The present invention generally relates to signal modulation, and particularly relates to imparting signal modulations using a switched-mode power amplifier.
In many fields, for example in third and higher generation base stations, bandwidth-optimized modulation schemes are used for transmitting information. Bandwidth-optimized modulation schemes require a non-constant envelope, and thus have a relatively high peak-to-average power ratio (PAR). Linear power amplifiers such as class AB amplifiers are typically used because they offer high linearity. However, class AB amplifiers must be driven with a high back-off to achieve good linearity across a wide operating range. Backing-off a class AB amplifier results in lower transmitted power, and thus reduced overall power efficiency. Class AB amplifiers must also be sized to handle peak power levels, but are often operated at much lower power levels. The efficiency of class AB amplifiers further suffers when operated below peak power levels.
Other conventional signal modulation techniques exist for Radio Frequency (RF) applications. However, each of the techniques suffers from poor power efficiency, poor linearity, complexity or other limitations. For example, supply voltage regulation techniques have poor power efficiency because the voltage regulator must have a large bandwidth. Linearity is problematic for Doherty amplifiers. Out-phasing, where two equally sized power amplifier outputs are combined via a power combiner, constantly dissipates power. Delta sigma modulators used in conjunction with a high-power output stage tend to be less efficient than their pulse-width modulator counterparts.
Pulse-width modulators conventionally drive a switched-mode power amplifier such as a class-D or class-J amplifier. Amplitude modulations are imparted at the amplifier output by varying the duty cycle of the pulse-width modulation signal applied to the amplifier input. However, switched-mode power amplifiers typically have a high-value inductor located between the amplifier output and the DC supply voltage for limiting RF currents at the DC power supply. The high supply-side inductance causes large voltage peaks at the drains of the amplifier switches, which can cause device damage. Moreover, the high supply-side inductance reduces the amplifier's reaction time to on-off and off-on transitions in the pulse-width modulation input control signal. This limits overall circuit performance because amplitude modulations are imparted based on how quickly the duty cycle of the pulse-width modulation control signal can be varied, i.e., how quickly the on-off/off-on transitions occur in the pulse-width modulation control signal.